The present invention relates to a high-frequency dielectric heating using a magnetron as in a microwave cooker and, more particularly, to a high-frequency dielectric heating, which is not influenced by differences such as the fluctuation or kind of the characteristics of the magnetron or the temperature of the anode of the magnetron.
In the conventional high-frequency heating apparatus, the electric power to be fed to the magnetron is adjusted with the output pulse width of an inverter control circuit. As the output voltage of signal superposing means becomes the higher, the output pulse width of the inverter control circuit becomes the larger so that the power to be fed to the magnetron becomes the higher. By this construction, the output voltage of the signal superposing means can be changed to change the heating output of the magnetron continuously.
On the other hand, the heater acts as the cathode of the magnetron, and a transformer for feeding the power to the magnetron feeds the power to the heater, too. Therefore, the power to be fed to the heater changes with the change in the power to be fed to the magnetron. If the heater temperature is to be confined within a proper range, therefore, only a small changing width of the heating output can be taken to raise a problem that the heating output cannot be continuously changed.
A high-frequency heating apparatus capable of solving that problem has been proposed as a control system, for example, in JP-A-7-176375. FIG. 12 is a diagram for explaining a high-frequency heating apparatus to execute that control system. In FIG. 12, the heating control system is constructed to comprise; a magnetron 701; a transformer 703 for feeding a high-voltage power to a high-voltage rectifying circuit 702 to feed a secondary winding power to the magnetron 701 and for feeding the power to a heater 715 of the magnetron 701; an inverter circuit 705 for rectifying an AC power source 704 to convert the rectified one to an alternative current of a predetermined frequency thereby to feed the converted current to the transformer 703; power detecting means 706 for detecting the input power or the output power of the inverter circuit 705; an output setting unit 707 for outputting an output set signal corresponding to a desired heating output setting; a power adjusting unit 708 for comparing the output of the power detecting means 706 and the output set signal to control the DC level of a power adjusting signal to a desired heating output; transmission detecting means 719 for changing a transmission detection signal as its output from LO to HI when the output of the power detecting means 706 exceeds an output level 718 of reference voltage generating means; a comparison voltage generating circuit 716 for generating a voltage corresponding to the output set signal; a waveform shaping circuit 721 for shaping the waveform shaping signal, as compared with the output set signal by a level converting circuit 720, and the output of the shaping circuit 710 for shaping the AC power source voltage 704, on the basis of the waveform shaping signal and the transmission detection signal; a comparator circuit 711 for comparing the output signal of the waveform shaping circuit 721 with the output of the comparison voltage generating circuit, to output a comparison reference voltage, when the former is lower, but to invert and amplify the former when the same is higher; signal superposing means 712 for superposing a fluctuating signal of the output of the comparator circuit 711 over the power adjusting signal, to output a pulse width control signal; an oscillation circuit 713; and an inverter control circuit 714 for modulating the output of the oscillation circuit 713 in pulse width with the pulse width control signal, to drive the inverter circuit 705 with the modulated output.
The aforementioned high-frequency heating apparatus adjusts the power to be fed to the magnetron 701, with the width of the output pulse of the inverter control circuit 714. As the output voltage of the signal superposing means 712 becomes the higher, the output pulse width of the inverter control circuit 714 becomes the wider, and the power to be fed to the magnetron 701 becomes the higher. In this apparatus, the heating output of the magnetron 701 can be continuously changed by changing the output voltage of the signal superposing means 712 continuously.
According to this construction, the shaping is done according to the output setting by the waveform shaping circuit 721 which inputs the shaping voltage of the AC power source 704 and outputs it to the comparator circuit 711. The output of the waveform shaping circuit 721 is inverted and amplified by the comparator circuit 711, which has as the reference voltage the comparison voltage generating circuit 716 for generating the reference signal at the level corresponding to the heating output setting signal. This inverted and amplified signal and the output of the power adjusting unit 708 are superposed so that the pulse width control signal, i.e., the output signal of the signal superposing means 712 takes a lower level near the maximum amplitude of the AC power source 704 when the heating output is set low than when set high. The level of the unoscillation portion of the magnetron becomes the higher so that the transmission period of the magnetron for one power source period becomes the longer. As a result, the power to be fed to the heater is raised. At the high output time, moreover, the input current waveform of the inverter takes such a waveform approximate the rectified waveform of the sine wave as is convex near the envelope peak, so that the higher harmonic current is suppressed.
Thus, the pulse width control signal is so controlled by the waveform shaping circuit 721 that a more heater current may flow in at the low output time and that the higher harmonics of the power source current may become smaller at the high output time. Therefore, it is possible to suppress the higher harmonics of the power source current and to reduce the change in the heater current thereby to realize a highly reliable high-frequency heating apparatus.
In this control, however, the ON/OFF drive pulse of the switching transistor is subjected to a pulse width modulation using the modulation waveform which is processed/shaped from the commercial power source waveform so that the waveform shaping by the “estimated control system” is realized to make the input current waveform resemble the sine wave. Therefore, it has been found out that the waveform shaping cannot follow so far as the fluctuation in the dispersion or kind of the characteristics of the magnetron, the fluctuation in the temperature of the anode of the magnetron and in the ebm (i.e., the anode-cathode voltage) due to the load in the microwave cooker, or the fluctuation in the power source voltage.
Here will be briefly described the dispersion or kind of the characteristics of the magnetron, which has motivated the invention. The VAK (i.e, the anode-cathode voltage)-Ib characteristics of the magnetron are covered by a nonlinear load, as shown in FIGS. 13A, 13B and 13C. In response to the phase of the commercial power source, therefore, the ON width is modulated to bring the input current waveform close to the sine wave thereby to improve the power factor.
Moreover, the nonlinear characteristics of the magnetron are different depending on the kinds of the magnetron and are fluctuated by the magnetron temperature or the load in the microwave cooker.
FIGS. 13A, 13B and 13C are characteristic diagrams of an anode-cathode applied voltage—an anode current of the magnetron. FIG. 13A is a diagram showing the differences due to the kinds of magnetrons; FIG. 13B is a diagram showing the differences due to the matching grade of the power supply of the magnetron; and FIG. 13C is a diagram showing the differences due to the temperature of the magnetron. Throughout FIGS. 13A to 13C, moreover, the ordinates indicate the anode-cathode voltage, and the abscissas indicate the anode current.
In FIG. 13A, therefore, letters A, B and C plot the characteristics of three kinds of magnetrons. In the case of the magnetron A, an electric current as low as IA1 or less flows till the VAK becomes VAK1 (=ebm). When the VAK exceeds the VAK1, however, the current IA abruptly begins to increase. In this region, the current IA abruptly changes for a small difference of the VAK. Next, in the case of the magnetron B, the VAK2 (=ebm) is lower than the VAK1. In the case of the magnetron C, moreover, the VAK3 (=ebm) is far lower than the VAK2. Thus, these nonlinear characteristics of the magnetron are different for the kinds A, B and C. In the case of the modulated waveform according to the magnetron having the low ebm, therefore, the input current waveform is distorted when a magnetron having a high ebm is used. These problems cannot be solved by the device of the prior art. Therefore, it is a target to provide a high-frequency dielectric heating circuit which is not subject to those influences of the kinds.
Turning to FIG. 13B, the characteristic diagrams of the three kinds of magnetrons show the impedance matching qualities of the heating chamber, as viewed from the magnetrons. In the case of a good impedance matching, the VAK1 (=ebm) takes the maximum and becomes the lower as the matching becomes the poorer. Thus, these nonlinear characteristics of the magnetron are seriously different for the impedance matching quality. Therefore, it is a target to provide a high-frequency dielectric heating circuit, which is not subject to those influences of the kinds.
Turning to FIG. 13C, the characteristic diagrams of the three kinds of magnetrons show the levels of the temperature of the magnetrons. In the case of a low temperature, the VAK1 (=ebm) takes the maximum and becomes the lower as the temperature becomes the higher. If the characteristics are adjusted to the lower temperature of the magnetron, therefore, the input voltage waveform is distorted when the temperature of the magnetron becomes high.
Thus, the nonlinear characteristics of the magnetron are seriously different for the difference in the temperature of the magnetron. Therefore, it is a target to provide a high-frequency dielectric heating circuit, which is not subject to those influences of the kinds.
Moreover, these fluctuations are not compensated in the circuit of the prior art and in the aforementioned circuit.